Association between Refractoriness to 131I
Therapy for Differentiated Thyroid Carcinoma
and 18F-FDG Accumulation in Lung Metastasis

Background: The purpose of this study was to retrospectively investigate the association between 2-[F-18]-fluoro-2-deoxy-D-glucose (¹⁸F-FDG) accumulation in lung metastasis (LM) before ¹³¹I therapy and refractoriness to ¹³¹I therapy for differentiated thyroid carcinoma (DTC) patients.

Methods and Findings: Sixty-one DTC patients with LM who underwent Positron emission tomography/computed tomography using ¹⁸F-FDG (¹⁸F-FDG PET/ CT) before an initial ¹³¹I therapy were retrospectively evaluated. Maximum of standardized uptake value (SUV_max) in LM with the highest ¹⁸F-FDG accumulation was measured in each patient. The SUV_max was compared between patients with and without ¹³¹I-positive LM, and between patients with and without an increased level of thyroglobulin (tumor marker) 12 ± 2 months after ¹³¹I therapy using the Wilcoxon test.

Discussion: Predictability for the patients with an increased thyroglobulin level was also analyzed by receiver-operating-characteristic (ROC) analysis. SUV_max of LM was significantly greater for patients without ¹³¹I-positive LM than for those with ¹³¹I-positive LM (5.9 ± 6.0 vs. 1.9 ± 2.0, p<0.01) and was significantly greater for patients with an increased level of TG after ¹³¹I therapy than for those without (7.0 ± 4.9 vs. 1.2 ± 1.0, p<0.01). All 11 of the 49 patients with SUV_max > 3.8 showed an increased TG level after ¹³¹I therapy. Use of the optimal cutoff threshold for SUV_max of 1.6 differentiated patients with an increased level of TG from those without at a sensitivity of 74.2%, a specificity of 94.4%, an accuracy of 81.6% and an AUC of 0.91.

Conclusion: ¹⁸F-FDG accumulation in LM from DTC can be one of predictors for refractoriness to ¹³¹I therapy.